1. Field of the Invention
The present invention relates to a technique for a light emitting device, more specifically, the invention relates to a driving method of the light emitting device.
2. Description of the Related Art
Recently, display devices for performing image display have been developed. Liquid crystal display devices that perform image display by using liquid crystal elements are widely used as display devices for mobile phones and personal computers because of advantages of high image quality, thinness, lightweight, and the like.
On the other hand, light emitting devices using the light emitting elements also have been developed in last years. Since the light emitting device needs no backlight, in addition to advantages of low power consumption, compact, lightweight, the light emitting device has characteristics of, for example, a high response speed suitable for moving image display, wide view, and thus, attracts a great deal of attention as flat panel display using for next generation small-size mobiles, which is available for full color moving image contents.
The light emitting element is constituted by a wide variety of materials, such as an organic material, an inorganic material, a thin film material, a bulk material, a dispersion material and so on. An organic light emitting diode (OLED) essentially constituted by an organic material can be an example of a typical light emitting element. The light emitting element has a structure of an anode, a cathode, and a light emitting layer sandwiched between the anode and cathode. The light emitting layer is constituted by one or more materials selected from the above materials.
A current flowing to the light emitting element is in directly proportional to the brightness of the light emitting element, the light emitting element emits light corresponding to an amount of the current flowing to the light emitting layer.
Incidentally, as driving methods used in displaying a multi-gradation image on a light emitting device, an analog gradation method and a digital gradation method are given. The former analog gradation method is a method in which a current is flown to the light emitting element corresponding to a desired gradation and the gradation is represented based on the magnitude of the current. The latter digital gradation method is a method in which the light emitting element is driven only in two states thereof: an ON state (state where the brightness is substantially 100%) and an OFF state (state where the brightness is substantially 0%).
Further, as driving methods for displaying multi-gradation images on the light emitting device, a voltage input method and a current input method are given. The former voltage input method is a method in which: a video signal (voltage) that is input to a pixel is input to a gate electrode of a driving element; and the driving element is used to control the brightness of a light emitting element. The latter current input method is a method in which the set signal current is flown to a light emitting element to control the brightness of the light emitting element. Both the analog gradation method and digital gradation method can be applied to the voltage input method and the current input method.
In order to provide a display device and a driving method thereto, which are capable of improving operation reliability of the light emitting element, a method of reducing light emission time of the pixel is given. (Refer to patent document 1)
[Patent Document 1] Patent Publication No. 2000-347622
The operations of a light-emitting device, to which the above-described analog gradation method is applied, will be described in reference to the timing chart of FIG. 7. In the timing chart of FIG. 7, the horizontal axis shows time and the vertical axis shows rows of the scanning line.
In the analog gradation method, as shown in FIG. 7, one frame period (F) is divided into: an addressing period (Ta) during which a video signal is written into a pixel; and a sustaining period (Ts) during which the pixel emits light in response to the video signal. The addressing period (Ta) and sustaining period (Ts) arise alternately, as time passes. In this case, the period during which each pixel emits light occupies much of one frame period. Therefore, each pixel emits light almost continuously unless the “black” video signal is input.